Method and system for a low input voltage low impedance termination stage for current inputs

ABSTRACT

Methods and systems for a low input voltage low impedance termination stage for current inputs may include, in a semiconductor die, generating an output current proportional to an input signal, where the output current is generated by an output stage that may include a pair of input cascode transistors and at least one pair of stacked output transistors. A source-follower feedback path for the input cascode transistors may include a feedback transistor with its gate terminal coupled to a drain terminal of a first of the input cascode transistors, a drain of the feedback transistor coupled to a supply voltage, and a source terminal of the feedback transistor coupled to a current source that is coupled to ground. A current source may be coupled to the drain of the first of the input cascode transistors. The supply voltage may be coupled to the stacked output transistors via a load resistor.

CROSS-REFERENCE TO RELATED APPLICATIONS/INCORPORATION BY REFERENCE

This application makes reference to and claims priority to U.S.Provisional Application Ser. No. 61/870,889 filed on Aug. 28, 2013. Theabove identified application is hereby incorporated herein by referencein its entirety.

FIELD

Certain embodiments of the invention relate to semiconductor devices.More specifically, certain embodiments of the invention relate to amethod and system for a low input voltage low impedance terminationstage for current inputs.

BACKGROUND

Complementary metal oxide semiconductor (CMOS) transistors areubiquitous in today's electronics devices. As CMOS trends continue toscale down the device feature size, handling high voltage/power levelsbecomes very challenging.

Further limitations and disadvantages of conventional and traditionalapproaches will become apparent to one of skill in the art, throughcomparison of such systems with the present invention as set forth inthe remainder of the present application with reference to the drawings.

BRIEF SUMMARY

A system and/or method for a low input voltage low impedance terminationstage for current inputs substantially as shown in and/or described inconnection with at least one of the figures, as set forth morecompletely in the claims.

Various advantages, aspects and novel features of the present invention,as well as details of an illustrated embodiment thereof, will be morefully understood from the following description and drawings.

BRIEF DESCRIPTION OF SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 illustrates an example electronic system that may be operable toperform digital-to-analog conversions, in accordance with an exampleembodiment of the disclosure.

FIG. 2 illustrates an example current-steering based digital-to-analogconvertor (DAC) architecture with low input voltage low impedancetermination stage for current input, in accordance with an exampleembodiment of the disclosure.

FIG. 3 illustrates an example power amplifier architecture with lowinput voltage low impedance termination stage for current input, inaccordance with an example embodiment of the disclosure.

FIG. 4 illustrates an example current-steering based digital-to-analogconvertor (DAC) architecture having a low input voltage and lowimpedance termination stage with impedance matching, in accordance withan example embodiment of the disclosure.

FIG. 5 illustrates an example power amplifier architecture having a lowinput voltage and low impedance termination stage with impedancematching, in accordance with an example embodiment of the disclosure.

DETAILED DESCRIPTION

Certain aspects of the disclosure may be found in low input voltage lowimpedance termination stage for current inputs. Exemplary aspects of theinvention may comprise an output stage for an electrical circuit, theoutput stage comprising a pair of input cascode transistors and at leastone pair of stacked output transistors. A source-follower feedback pathfor the input cascode transistors may comprise a feedback transistorwith its gate terminal coupled to a drain terminal of a first of theinput cascode transistors, a drain of the feedback transistor coupled toa supply voltage, and a source terminal of the feedback transistorcoupled to a current source that is coupled to ground. A current sourcemay be coupled to the drain of the first of the input cascodetransistors. The supply voltage may be coupled to the at least one pairof stacked output transistors via a load resistor. The input cascodetransistors, the feedback transistor, and the at least one pair ofstacked output transistors may comprise complementary metal-oxidesemiconductor (CMOS) transistors. A current through the input cascodetransistors may be controlled by a bias voltage applied to a gateterminal of the first of the input cascode transistors. The electricalcircuit may comprise a power amplifier. The power amplifier may comprisea differential gain stage coupled to a source terminal of the first ofthe cascode transistors and a drain of a second of the cascodetransistors. The electrical circuit may comprise a digital-to-analogconverter (DAC) that may comprise a plurality of unit cells, each cellcoupled to a source terminal of the first of the cascode transistors anda drain of a second of the cascode transistors. Each of the plurality ofunit cells may also be coupled to cascode transistors in a second outputstage nominally identical to the output stage. A feedback circuitcomprising a resistor and a capacitor in series may be coupled from thepair of input cascode transistors to a load resistance that is coupledto the at least one pair of stacked output transistors.

As utilized herein, “and/or” means any one or more of the items in thelist joined by “and/or”. As an example, “x and/or y” means any elementof the three-element set {(x), (y), (x, y)}. As another example, “x, y,and/or z” means any element of the seven-element set {(x), (y), (z), (x,y), (x, z), (y, z), (x, y, z)}. As utilized herein, the terms “block”and “module” refer to functions than can be implemented in hardware,software, firmware, or any combination of one or more thereof. Asutilized herein, the term “exemplary” means serving as a non-limitingexample, instance, or illustration. As utilized herein, the term “e.g.,”introduces a list of one or more non-limiting examples, instances, orillustrations.

FIG. 1 illustrates an example electronic system that may be operable toperform digital-to-analog conversions, in accordance with an exampleembodiment of the disclosure. Referring to FIG. 1, there is shownelectronic system 100.

The electronic system 100 may comprise suitable circuitry, interfaces,logic and/or code for implementing various aspects of the presentdisclosure. In this regard, the electronic system 100 may be configuredto support performing, executing or running various operations,functions, applications and/or services. For example, the electronicsystem 100 may be used for executing computer programs, playing videoand/or audio content, gaming, communication applications or services(e.g., Internet access and/or browsing, email, text messaging, chattingand/or voice calling services), and/or networking services (e.g., WiFihotspot, Bluetooth piconet, Ethernet networking, cable or satellitesystems, and/or active 4G/3G/femtocell data channels).

In some instances, the electronic system 100 may be configured to enableand/or support communication of data. In this regard, the electronicsystem 100 may need to communicate with other systems (local or remote),such as during executing, running, and/or performing of operations,functions, applications and/or services supported by the electronicsystem 100. For example, the electronic system 100 may be configured tosupport (e.g., using suitable dedicated communication components orsubsystems) use of wired and/or wireless connections/interfaces, whichmay be configured in accordance with one or more supported wirelessand/or wired protocols or standards, to facilitate transmission and/orreception of signals (carrying data) to and/or from the electronicsystem 100. In this regard, the electronic system 100 may be operable toprocess transmitted or received signals in accordance with applicablewired or wireless protocols.

Examples of wireless protocols or standards that may be supported and/orused by the electronic system 100 may comprise wireless personal areanetwork (WPAN) protocols, such as Bluetooth (IEEE 802.15); near fieldcommunication (NFC) standards; wireless local area network (WLAN)protocols, such as WiFi (IEEE 802.11); cellular standards, such as2G/2G+ (e.g., GSM/GPRS/EDGE, and IS-95 or cdmaOne) and/or 2G/2G+ (e.g.,CDMA2000, UMTS, and HSPA); 4G standards, such as WiMAX (IEEE 802.16) andLTE; Ultra-Wideband (UWB), and/or the like. Examples of wired protocolsand/or interfaces that may be supported and/or used by the electronicsystem 100 comprise Ethernet (IEEE 802.2), Fiber Distributed DataInterface (FDDI), Integrated Services Digital Network (ISDN), cabletelevision and/or internet (ATSC, DVB-C, DOCSIS), and Universal SerialBus (USB) based interfaces. Examples of signal processing operationsthat may be performed by the electronic system 100 comprise, forexample, filtering, amplification, analog-to-digital conversion and/ordigital-to-analog conversion, up-conversion/down-conversion of basebandsignals, encoding/decoding, encryption/decryption, and/ormodulation/demodulation.

In some instances, the electronic system 100 may be configured to enableor support input/output operations, such as to allow user interactionsthat may be needed for controlling services provided by the electronicsystem 100 (e.g., to allow operators to provide input or commands forcontrolling location specific marketing, or obtain output or feedbackpertaining to it). In this regard, the electronic system 100 maycomprise components or subsystems for enabling interactions with a user(e.g., end-user or installer), so as to obtain user input and/or toprovide user output.

The electronic system 100 may be a stationary system (i.e., beinginstalled at, and/or configured for use only in particular location). Inother instances, however, the electronic system 100 may be a mobiledevice—i.e., intended for use on the move and/or at different locations.In this regard, the electronic system 100 may be designed and/orconfigured (e.g., as handheld device) to allow for ease of movement,such as to allow it to be readily moved while being held by the user asthe user moves, and the electronic system 100 may be configured toperform at least some of the operations, functions, applications and/orservices supported on the move.

Examples of electronic systems may comprise handheld electronic devices(e.g., cellular phones, smartphones, or tablets), personal computers(e.g., laptops or desktops), servers, dedicated multimedia devices(e.g., televisions, game consoles, or portable media players), set-topboxes (STBs) or other similar receiver systems, and the like. Thedisclosure, however, is not limited to any particular type of electronicsystem.

In operation, the electronic system 100 may be operable to performvarious operations, functions, applications and/or services. Forexample, in some instances, electronic system 100 may be configured orused to communicate data (to and/or from the system), and to process thecommunicated data. In this regard, communication of data, whether overwired or wireless interfaces, may typically comprise transmitting and/orreceiving analog signals that are communicated over wireless and/orwired connections. In this regard, typically analog radio frequency (RF)signals may be used to carry data (e.g., content), which may be embeddedinto the analog signals using analog or digital modulation schemes. Foranalog communications, data is transferred using continuously varyinganalog signals, and for digital communications, the analog signals areused to transfer discrete messages in accordance with a particulardigitalization scheme. Accordingly, handling of digital communications(e.g., in the electronic system 100) may typically require performing,inter alia, digital-to-analog conversions at the transmitting end andanalog-to-digital conversions at the receiving end.

For example, the electronic device may comprise one or moredigital-to-analog converters (DACs) 110. In this regard, each DAC 110may comprise circuitry, interfaces, logic and/or code for performingdigital-to-analog conversions.

In this regard, each DAC 110 may comprise suitable circuitry,interfaces, logic, and/or code for performing digital-to-analogconversions. The DAC 110 may be utilized, for example, during signalprocessing, such as to allow converting digital data into analogwaveforms that may be embedded into transmitted radio frequency (RF)signals. The disclosure, however, is not limited to any particular usescenario, and may be utilized in any appropriate setup performing orrequiring digital-to-analog conversions.

Various architectures and/or designs may be used in implementingdigital-to-analog converters (DACs). For example, DACs may beimplemented based on current-steering. In this regard, incurrent-steering DACs, the conversion from digital to analog may beperformed based on steering of current from sources, with the steeringbeing controlled or adjusted based on the input digital code.

There may be certain issues and/or disadvantages with use ofcurrent-steering DACs, however. For example, typically current-steeringDACs may be terminated in a diode connected transistor when furtheramplification is desired. Current flowing through the diode connectedtransistor may be then mirrored and scaled up for amplification. Thedisadvantage of such termination is that it may reduce the voltage dropacross the current-steering DAC stage giving rise to various DACnon-idealities. Also the signal voltage swing due to signal currentflowing in the diode connected transistor may give rise to theundesirable signal dependent DAC switching operation. Another issue withthe diode connected termination is achieving large current gain. Whenhigh current gain is desired, the current mirror bandwidth may sufferdue to large capacitive loading at the diode connected termination.Accordingly, in various implementations in accordance with the presentdisclosure, the problems described above may be solved and/or remedied.For example, these problems may be addressed with an introduction of lowinput impedance termination stage. An example implementationincorporating a low input impedance termination stage is described inmore detail with respect to FIG. 2.

FIG. 2 an example current-steering based digital-to-analog convertor(DAC) architecture with low input voltage low impedance terminationstage for current input, in accordance with an example embodiment of thedisclosure. Referring to FIG. 2, there is shown a digital-to-analogconvertor (DAC) 200.

The DAC 200 may comprise suitable circuitry, interfaces, logic, and/orcode for performing digital-to-analog conversions. The DAC 200 may besubstantially similar to the DAC 100 of FIG. 1. The DAC 200 may beimplemented based on current-steering architecture.

In this regard, the DAC 200 may comprise a plurality of DAC unit cells210 and a pair of termination stage circuits 220 _(A) and 220 _(B). Thetermination stage circuits 220 _(A) and 220 _(B) may be nominallyidentical, within normal device tolerances, for example. Any number ofDAC unit cells may be utilized, depending on die size limitations andthe desired bit resolution for the DAC, for example. Each DAC unit cell210 may comprise a differential load steering circuit and a decoderlatch 212. The differential load steering circuit may comprise aplurality of transistors M_(L1) and M_(L2) (e.g., PMOS transistors),which may be configured in differential load manner—e.g., to a supplyvoltage V_(dd), in a controlled manner, through two switching branchescomprising the transistors M_(S1) and M_(S2) (e.g., PMOS transistors).

The decoder latch 212, which may be adapted to generate control signalsto the steering circuit based on input code (i.e., the digital codebeing converted). In this regard, the decode latch 212 may open onebranch by enabling M_(S1) or M_(S2) (while closing the other branch)based on the binary input (logic ‘0’ or ‘1’).

The termination stage circuits 220 _(A) and 220 _(B) may be configuredfor providing source follower staging. In this regard, use of thetermination stage circuits 220 _(A) and 220 _(B) may enhance performanceof the DAC 200, such as by lowering termination impedance and/orlowering CM voltage for DAC output. The use of termination stagecircuits 220 _(A) and 220 _(B) may also allow for use of current sourcesthat may only be a fraction of full scale DAC currents. In the exampleimplementation of the termination stage circuits in the DAC 200, theoutput of the current-steering DAC may be terminated into the source ofcascode transistors (e.g., NMOS) M2 of termination stage circuits 220_(A) and 220 _(B). Transistors (e.g., NMOS) M3 with bias current sourcesI_(S), form a feedback path for M1, may comprise a source follower stagethat buffers the large capacitive loading at node B due to large currentscaling ratio.

While NMOS transistors are shown in FIG. 2, the disclosure is not solimited. Accordingly, PMOS transistors may also be utilized in a circuitthat may essentially be a mirror image of DAC 200.

For example, with respect to the termination stage circuit 220 _(A), theinput impedance (Z_(in)) into node A may be given by the equation:

$\begin{matrix}{Z_{in} = \frac{1}{g_{m\; 1}( {g_{m\; 2} \cdot r_{o\; 2}} )}} & (1)\end{matrix}$

In comparison, for a diode connected transistor termination stage, theinput impedance is 1/g_(m1). The new termination stage may show areduction in the input impedance by factor of g_(m2)*r_(o2) (˜25). Sincethe input impedance is reduced by at least an order of magnitude, thesignal swing at this node may also be reduced by the same factor.

The DC voltage at node A may be controlled by using bias voltage Vb. Inthis regard, a minimum voltage at node A may be V_(dsat1), thesaturation voltage of transistor M1. This may be an improvement comparedto a diode connected transistor stage, which has a voltage drop ofVT+V_(dsat), and may result in an additional 500 mV headroom for thecurrent-steering DAC 200 to operate.

The impedance into node B (Z_(B)) may be given by the equation:

$\begin{matrix}{Z_{B} = \frac{1}{{g_{m\; 3}( {g_{m} \cdot r_{o}} )}^{2}}} & (2)\end{matrix}$

Compared to a diode connected termination stage, this is an improvementby a factor of (g_(m)*r_(o))². This may improve the bandwidth for largecurrent mirror scaling ratios. The feedback loop around M1-M3 mustgenerally be stable for correct operation of the termination stage. Itcan be easily compensated for by using a capacitor across drain andsource nodes of transistor M2. Currents I_(B) and I_(S) can be afraction of the full-scale DAC currents, resulting in a small increasein power dissipation compared to a diode connected termination stage.

The resistors R_(Load) in the termination stage circuits 220 _(A) and220 _(B) may represent the output load of the current steering DAC 200,as shown by the differential output voltage V_(op)−V_(on). The currentI_(B) may be mirrored by a plurality of stages, as indicated by thedashed line extending from node A, with the stages comprising stackedtransistors such as M4 and M5 (e.g., NMOS). Any number of stackedtransistors may be utilized to provide a desired number of current stepsfrom the minimum current, through M1, to the maximum current, suppliedby the last transistor M5 with a size ratio of N:1 over the transistorM1.

While the termination stage circuits 220 _(A) and 220 _(B) are shown astermination stages for a DAC in FIG. 2, the disclosure is not solimited. Accordingly, the termination stage circuits 220 _(A) and 220_(B) may be used in any application where low output impedance and lowvoltage is desired or required, such as a power amplifier, for example.

FIG. 3 an example power amplifier architecture with low input voltagelow impedance termination stage for current input, in accordance with anexample embodiment of the disclosure. Referring to FIG. 3, there isshown a power amplifier 300. The power amplifier 300 may comprise a gainstage 303 and termination stage circuits 320A and 320B, and may beoperable to provide gain to an input signal 301.

The termination stage circuits 320 _(A) and 320 _(B) may be configuredfor providing source follower staging. In this regard, use of thetermination stage circuits 320 _(A) and 320 _(B) may enhance performanceof the power amplifier 300, such as by lowering termination impedanceand/or lowering CM voltage for amplifier output. The use of terminationstage circuits 320 _(A) and 320 _(B) may also allow for use of currentsources that may only be a fraction of full scale amplifier currents. Inthe example implementation of the termination stage circuits in thepower amplifier 300, the output of the gain stage 303 may be terminatedinto the source of cascode transistors (e.g., NMOS) M2 of terminationstage circuits 320 _(A) and 320 _(B). Transistors (e.g., NMOS) M3 withbias current sources I_(S), form a feedback path for M1, may comprise asource follower stage that buffers the large capacitive loading at nodeB due to large current scaling ratio.

The resistors R_(Load) in the termination stage circuits 320 _(A) and320 _(B) may represent the output load of the power amplifier 300, asshown by the differential output voltage V_(op)−V_(on). The currentI_(B) may be mirrored by a plurality of stages, as indicated by thedashed line extending from node A, with the stages comprising stackedtransistors such as M4 and M5 (e.g., NMOS). Any number of stackedtransistors may be utilized to provide a desired number of current stepsfrom the minimum current, through M1, to the maximum current, suppliedby the last transistor M5 with a size ratio of N:1 over the transistorM1.

It should again be noted that while NMOS transistors are shown in FIG.3, the disclosure is not so limited. Accordingly, PMOS transistors mayalso be utilized in a circuit that may essentially be a mirror image ofpower amplifier 300.

FIG. 4 illustrates an example current-steering based digital-to-analogconvertor (DAC) architecture having a low input voltage and lowimpedance termination stage with impedance matching, in accordance withan example embodiment of the disclosure. In many configurations, thetermination circuit may need to provide a well-defined impedance to bematched to an external load. In such applications, the value of theoutput load impedance R_(Load) in the termination stage circuits 220_(A) and 220 _(B), or circuits 320 _(A) and 320 _(B) may be determinedby these constraints. In many configurations, this impedance is verysmall such as, for example, of the order of 50 or 75 ohms. With such lowvalues of output impedance, to meet a certain voltage swing, the outputstage current would need to be increased, leading to high powerconsumption.

This larger current requirement may be mitigated by the introduction ofa feedback resistor capacitor network connecting the termination stagecircuits 420 _(A) and 420 _(B) to the plurality of DAC unit cells 410,using the resistor R_(FB) and the capacitor C_(FB). The capacitor C_(FB)may be used for AC coupling.

It can be shown mathematically that in such a configuration, the outputimpedance provided by the termination stage circuits 420 _(A) and 420_(B) may be given by:

$Z_{BOUT} = {R_{LOAD}{\frac{R_{FB}}{N + 1}}}$Where “∥” denotes the “parallel” configuration, indicating two resistorsin parallel. In this circuit, the value of resistor R_(FB) and R_(LOAD)may be increased to a large value, as long as a large value of N is usedin the circuit, while still meeting the output impedance matchingrequirement. Under a matched condition, RLOAD=RFB/(N+1). Since anadditional parallel RLOAD resistor is not required for matching, the DCcurrent consumed by the output stage may be reduced by half, which leadsto considerably lower power consumption.

FIG. 5 illustrates an example power amplifier architecture having a lowinput voltage and low impedance termination stage with impedancematching, in accordance with an example embodiment of the disclosure.FIG. 5 shows a similar modification to the circuit of FIG. 3 (e.g.,similar to the modification shown by FIG. 4), where a feedback pathcomprising R_(FB) and C_(FB) has been added for impedance matching. Aswith the circuit in FIG. 4, the output impedance may be matched with areceiving load circuit without an additional parallel load resistor,decreasing the DC current by half.

In an embodiment of the disclosure, a method and system may comprise anoutput stage for an electrical circuit, the output stage comprising apair of input cascode transistors and at least one pair of stackedoutput transistors. A source-follower feedback path for the inputcascode transistors may comprise a feedback transistor with its gateterminal coupled to a drain terminal of a first of the input cascodetransistors, a drain of the feedback transistor coupled to a supplyvoltage, and a source terminal of the feedback transistor coupled to acurrent source that is coupled to ground.

A current source may be coupled to the drain of the first of the inputcascode transistors. The supply voltage may be coupled to the at leastone pair of stacked output transistors via a load resistor. The inputcascode transistors, the feedback transistor, and the at least one pairof stacked output transistors may comprise complementary metal-oxidesemiconductor (CMOS) transistors. A current through the input cascodetransistors may be controlled by a bias voltage applied to a gateterminal of the first of the input cascode transistors.

The electrical circuit may comprise a power amplifier. The poweramplifier may comprise a differential gain stage coupled to a sourceterminal of the first of the cascode transistors and a drain of a secondof the cascode transistors. The electrical circuit may comprise adigital-to-analog converter (DAC) that may comprise a plurality of unitcells, each cell coupled to a source terminal of the first of thecascode transistors and a drain of a second of the cascode transistors.Each of the plurality of unit cells may also be coupled to cascodetransistors in a second output stage nominally identical to the outputstage. A feedback circuit comprising a resistor and capacitor in seriesmay be coupled from the pair of input cascode transistors to a loadresistance that is coupled to the at least one pair of stacked outputtransistors.

Other embodiments may provide a non-transitory computer readable mediumand/or storage medium, and/or a non-transitory machine readable mediumand/or storage medium, having stored thereon, a machine code and/or acomputer program having at least one code section executable by amachine and/or a computer, thereby causing the machine and/or computerto perform the steps as described herein for low input voltage lowimpedance termination stage for current inputs.

Accordingly, aspects of the invention may be realized in hardware,software, firmware or a combination thereof. The invention may berealized in a centralized fashion in at least one computer system or ina distributed fashion where different elements are spread across severalinterconnected computer systems. Any kind of computer system or otherapparatus adapted for carrying out the methods described herein issuited. A typical combination of hardware, software and firmware may bea general-purpose computer system with a computer program that, whenbeing loaded and executed, controls the computer system such that itcarries out the methods described herein.

One embodiment may be implemented as a board level product, as a singlechip, application specific integrated circuit (ASIC), or with varyinglevels integrated on a single chip with other portions of the system asseparate components. The degree of integration of the system willprimarily be determined by speed and cost considerations. Because of thesophisticated nature of modern processors, it is possible to utilize acommercially available processor, which may be implemented external toan ASIC implementation of the present system. Alternatively, if theprocessor is available as an ASIC core or logic block, then thecommercially available processor may be implemented as part of an ASICdevice with various functions implemented as firmware.

The present invention may also be embedded in a computer programproduct, which comprises all the features enabling the implementation ofthe methods described herein, and which when loaded in a computer systemis able to carry out these methods. Computer program in the presentcontext may mean, for example, any expression, in any language, code ornotation, of a set of instructions intended to cause a system having aninformation processing capability to perform a particular functioneither directly or after either or both of the following: a) conversionto another language, code or notation; b) reproduction in a differentmaterial form. However, other meanings of computer program within theunderstanding of those skilled in the art are also contemplated by thepresent invention.

While the invention has been described with reference to certainembodiments, it will be understood by those skilled in the art thatvarious changes may be made and equivalents may be substituted withoutdeparting from the scope of the present invention. In addition, manymodifications may be made to adapt a particular situation or material tothe teachings of the present invention without departing from its scope.Therefore, it is intended that the present invention not be limited tothe particular embodiments disclosed, but that the present inventionwill include all embodiments falling within the scope of the appendedclaims.

What is claimed is:
 1. A semiconductor device, the device comprising: anoutput stage for an electrical circuit, the output stage comprising apair of input cascode transistors and at least one pair of stackedoutput transistors, wherein a source-follower feedback path for theinput cascode transistors comprises a feedback transistor with its gateterminal coupled to a drain terminal of a first of the input cascodetransistors, a drain of the feedback transistor coupled to a supplyvoltage, and a source terminal of the feedback transistor coupled to acurrent source that is coupled to ground.
 2. The semiconductor deviceaccording to claim 1, wherein a current source is coupled to the drainof the first of the input cascode transistors.
 3. The semiconductordevice according to claim 1, wherein the supply voltage is coupled tothe at least one pair of stacked output transistors via a load resistor.4. The semiconductor device according to claim 1, wherein the inputcascode transistors, the feedback transistor, and the at least one pairof stacked output transistors comprise complementary metal-oxidesemiconductor (CMOS) transistors.
 5. The semiconductor device accordingto claim 1, wherein a current through the input cascode transistors iscontrolled by a bias voltage applied to a gate terminal of the first ofthe input cascode transistors.
 6. The semiconductor device according toclaim 1, wherein the electrical circuit comprises a power amplifier. 7.The semiconductor device according to claim 6, wherein the poweramplifier comprises a differential gain stage coupled to a sourceterminal of the first of the cascode transistors and a drain of a secondof the cascode transistors.
 8. The semiconductor device according toclaim 1, wherein the electrical circuit comprises a digital-to-analogconverter (DAC).
 9. The semiconductor device according to claim 8,wherein the DAC comprises a plurality of unit cells, each cell coupledto a source terminal of the first of the cascode transistors and a drainof a second of the cascode transistors.
 10. The semiconductor deviceaccording to claim 9, wherein each of the plurality of unit cells arealso coupled to cascode transistors in a second output stage nominallyidentical to the output stage.
 11. The semiconductor device according toclaim 1, wherein a feedback circuit comprising a resistor and capacitorin series is coupled from the pair of input cascode transistors to aload resistance that is coupled to the at least one pair of stackedoutput transistors.
 12. A method for communication, the methodcomprising: in a semiconductor die, generating an output current that isproportion to an input signal, said output current generated by anoutput stage, the output stage comprising: a pair of input cascodetransistors and at least one pair of stacked output transistors; and asource-follower feedback path for the input cascode transistors, saidsource-follower feedback path comprising: a feedback transistor with itsgate terminal coupled to a drain terminal of a first of the inputcascode transistors; a drain of the feedback transistor coupled to asupply voltage; and a source terminal of the feedback transistor coupledto a current source that is coupled to ground.
 13. The method accordingto claim 12, wherein a current source is coupled to the drain of thefirst of the input cascode transistors.
 14. The method according toclaim 12, wherein the supply voltage is coupled to the at least one pairof stacked output transistors via a load resistor.
 15. The methodaccording to claim 12, wherein the input cascode transistors, thefeedback transistor, and the at least one pair of stacked outputtransistors comprise complementary metal-oxide semiconductor (CMOS)transistors.
 16. The method according to claim 12, comprisingcontrolling a current through the input cascode transistors with a biasvoltage applied to a gate terminal of the first of the input cascodetransistors.
 17. The method according to claim 12, wherein theelectrical circuit comprises a power amplifier.
 18. The method accordingto claim 17, wherein the power amplifier comprises a differential gainstage coupled to a source terminal of the first of the cascodetransistors and a drain of a second of the cascode transistors.
 19. Themethod according to claim 12, wherein the electrical circuit comprises adigital-to-analog converter (DAC).
 20. The method according to claim 19,wherein the DAC comprises a plurality of unit cells, each cell coupledto a source terminal of the first of the cascode transistors and a drainof a second of the cascode transistors and wherein each of the pluralityof unit cells are also coupled to cascode transistors in a second outputstage nominally identical to the output stage.
 21. The method accordingto claim 12, wherein a feedback circuit comprising a resistor andcapacitor in series is coupled from the pair of input cascodetransistors to a load resistance that is coupled to the at least onepair of stacked output transistors.
 22. A semiconductor devicecomprising: a pair of output stages for a digital-to-analog converter(DAC), the output stages each comprising: a pair of input cascodetransistors; at least one pair of stacked output transistors; a feedbackpath comprising a resistor and capacitor in series coupled from the pairof input cascode transistors to a load resistance that is coupled to theat least one pair of stacked output transistors; and a source-followerfeedback path for the input cascode transistors, said source-followerfeedback path comprising: a feedback transistor with its gate terminalcoupled to a drain terminal of a first of the input cascode transistors;a drain of the feedback transistor coupled to a supply voltage; and asource terminal of the feedback transistor coupled to a current sourcethat is coupled to ground.